Duoplasmatron source having an offset compressor electrode



March 12, 1968 yL. E. COLLINS ETAL.

DUOPLASMATRON SOURCE HAVING AN OFFSET COMPRESSOR ELECTRODE 3sheets-'sheet 1 Filed Oct. 18, 1965 March 12, 1968 L. E. COLLINS ETAL 5Sheets-Sheet 2 Filed Oct. 18, 1965 linfa March 12, 1968 L. E. COLLINSETAL 3,373,305

DUOPLASMATRON SOURCE HAVING AN OFFSET COMPRESSOR ELECTHODE Filed OC..18, 1965 5 Sheets-Sheet 3 United States Patent C 3,373,305 DUQPLASMATRONSOURCE HAVING AN OFFSET COMPRESSOR ELECTRDE Leslie Edward Collins,Reading, and Royston Henry Gobbett, Basingstoke, England, assignors toUnited Kingdom Atomic Energy Authority, London, England Filed Oct. 18,1965, Ser. No. 497,248 Claims priority, application Great Britain, Oct.26, 1964, 43,658/ 64 3 Claims. (Cl. 313-230) ABSTRACT F THE DISCLOSURE Aduoplasmatron source for producing negative ions including a compressorhaving an orifice, an anode having an orifice and an extractor having anorifice. The axis of the compressor orice is displaced from the commonaxis of the anode and the extractor orifices to allow negative ions fromthe outer region of an arc discharge established between compressor andanode to be extracted through the anode orifice.

This invention relates to ion sources and in particular to sources ofthe duoplasmatron type.

A full description of a duoplasmatron source can be found, for example,in Rev. Sci. Instrum., vol. 30, p. 694 (1959). Briefiy it comprises acompressor electrode having an outlet orifice, an electron-emittingfilament located behind the compressor electrode, an anode located infront of the compressor electrode and having a small orifice coaxialwith the outlet orifice thereof, and an extractor electrode locatedbeyond the anode orifice. An axial magnetic field is applied in thespace between cornpressor and anode. In operation, electrons from thefilament bombard gas behind the compressor to produce a weak plasmawhich passes through the compressor orifice to the gap betweencompressor and anode, in which a low-voltage arc discharge is struck.This arc is com- Ipressed by the magnetic field to produce a denseplasma, ions from which pass through the anode orice and are drawn ottby a suitable potential applied to the extractor.

The duoplasmatron source is normally used as a source of positive ions,the extractor being made negative with respect to the anode. For somepurposes negative ions are required, but the mere application of apositive eX- traction voltage in existing duoplasmatron sources producespoor results, with a high ratio of electrons to negative ions in theresulting beam.

The present invention provides a duoplasmatron source for the productionof negative ions wherein the anode orifice is so oriented in relation tothe arc discharge established between compressor and anode as to causenegative ions extracted from said orifice to be drawn from an outerregion of said discharge.

The present invention also provides a duoplasmatron source for theproduction of negative ions including a compressor having an orifice andan anode having an orifice, wherein the axis of the compressor orificeis displaced rom the axis of the anode orifice to allow negative ionsfrom the outer region of an arc discharge established between compressorand anode to be extracted through said anode orifice.

To enable the nature of the present invention to be more readilyunderstood, attention is directed, by way of example, to theaccompanying drawing wherein:

FIGURE l is a sectional elevation of the electrode structure of an ionsource embodying the present invention.

FIGURE 2 is an enlarged sectional elevation of an anode insert used inthe embodiment of FIGURE 1.

ice

FIGURES 3-5 are graphs showing the performance of an ion source withvarious parameters.

FIGURE 6 is a sectional elevation of an embodiment similar to that shownin FIGURE l.

FIGURE 1 shows the filament 1, compressor 2, anode 3 and extractor 4 ofa duoplasmatron source. In prior sources of this type, the compressororifice 5, the anode orifice 6 and the extractor orifice 7 are coaxialwith one another. In the present source, however, the axis of the anodeand extractor orifices are displaced by a distance d from the axis ofthe compressor electrode. As in known practice, the anode 3 is not asimple metal sheet as shown in FIGURE l, but is made of copper having atungsten insert 8 (FIGURE 2) in which the orifice 6 is formed. Thiscomposite form of construction is not, however, essential. The axialmagnetic field is applied in a known manner by an externalcircumferential winding shown symbolically at 9.

FIGURE 6 shows the construction of the embodiment to which the graphs ofFIGURES 35 relate, in which the tungsten insert 8 is recessed a distanceg into the anode 3. The remaining reference numerals and letterscorrespond to those in FIGURES l and 2, except that hole 6 and dimensiona are not marked.

Dimensions which have proved suitable for the production of negativehydrogen ions, referring to the letters shown in FIGURES 1, 2 and 8 are:

a=0.03 inch b=0.l inch c=0.1 inch d=0.0625 inch e=0.2 inch f=0.12 inchg=0.06 inch (FIGURE 6 only) A positive voltage with respect to the anode3 is applied to the extractor 4.

FIGURE 3 shows how the negative ion yield was found to vary withdimension d above, the several curves being for different values ofdimension f. It will be seen that the optimum yield was obtained withd=0.0625 inch and f=0.l2 inch.

FIGURE 4 shows the variation ofthe electron/negative-ion current ratioin the beam as d is varied, for f=0.12 inch. At peak ion current theelectron current is increasing more rapidly than the ion current withdecreasing d, and hence the above ratio also increases as d decreases.However even at peak ion yield (1120.0625 inch), the ratio is only about2 to l, which compares favourably with values of about 1000 to 1reported in the literature. The electron current collected by theextractor at peak ion yield is about 4-5 ma.

The optimum arc discharge current for negative ion production was -foundto be about 12 amps, but was not very critical, nor was the 4gaspressure which showed a flat peak at l mm. Hg.

The effect of varying the axial magnetic field is shown in FIGURE 5. Theresults shown in FIGURE 3 indicate that the arc discharge contains anannular outer region possessin-g optimum conditions for negative ionproduction. It would therefore be expected that if d were decreased froman initial value, the axial field would have to be increased to applyfurther compression to the arc, so that the annular region remainedaligned with the anode orifice, in order to restore the initial ionyield. In fact, however, the field current can be adjusted to give anincreased ion yield as d is decreased. FIGURE 5 shows the magnetic fieldcurrent required to give maximum yield as d is decreased.

We claim:

1. A duoplasmatron source for the production of negative ions includinga compressor having an orice and an anode and 'an extractor each havin-gan orifice, wherein the axis of the compressor oriiice is displaced fromthe common axis of the anode and extractor orices' to allow negativeions from th'e outer region of an arc discharge established betweencompressor and anode to be extracted through said anode oriice.

2. A duoplasmatron source for the production of negative ions includinga compressor, an 'anode and an extractor each having an orice, whereinthe anode and extractor orices have a common axis, the axis of t-hecompressor orifice being displaced from said common axis in a directionnormal to said common axis and said anode orifice being substantiallysmaller than said compresser orice so that negative ions from the outerregion of an arc discharge established between compressor and anode maybe extracted from said discharge.

3. In a duoplasmatron sour-ce comprising a compressor having an orice'and an anode and an extractor having an orilce, the displacement of theaxis of the cornpressor oriiice from the common axis of the anode a'ndextractor orices in a direction normal to said axes t allow theextraction of negative ions from t-he outer region of an 'arc dischargeestablished between compresser and anode.

. References Cited UNITED STATES PATENTS 2,816,243 10/ 1957 Herb 313--632,978,580 4/ 1961 Von Ardenne 313-230 JAMES w. LAWRENCE, PrimaryExaminer.

R. JUDD, Assistant Examiner.

